In LTE (Long Term Evolution), OFDM (Orthogonal Frequency Division Multiplexing) is used in the downlink and DFT-spread (Discrete Fourier Transform) OFDM is used in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, where each radio frame consists of ten equally-sized subframes of length Tsubframe=1 ms, as illustrated in FIG. 2.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot of 0.5 ms in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled in LTE, i.e., in each subframe, a base station transmits control information concerning which mobile terminals data is transmitted to, and upon which resource blocks the data is transmitted in the current downlink subframe. Typically, this control signaling is transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. A downlink system with 4 OFDM symbols as control region is illustrated in FIG. 4.
In LTE, hybrid-ARQ is used, where, after receiving downlink data in a subframe, a mobile terminal attempts to decode it and reports to a base station whether the decoding was successful or not. When the decoding has been successful, the report comprises an “ACK” (ACKnowledgment), and when the decoding was not successful, the report comprises a “NAK” (Negative AcKnowledgment). In case of an unsuccessful decoding attempt, the base station can retransmit the erroneous data.
LTE uplink control signaling from a mobile terminal to a base station comprises:                hybrid-ARQ acknowledgements for received downlink data;        mobile terminal reports related to the downlink channel conditions, to be used as assistance for the downlink scheduling; and        scheduling requests, indicating that a mobile terminal needs uplink resources for uplink data transmissions.        
When a mobile terminal has not been assigned an uplink resource for data transmission, the L1/L2 (Layer1/Layer2) control information, i.e., channel-status reports, hybrid-ARQ acknowledgments and scheduling requests, is transmitted in uplink resources, i.e. resource blocks, specifically assigned for uplink L1/L2 control on a Physical Uplink Control CHannel (PUCCH). As illustrated in FIG. 6, these resources are located at the edges of the total available cell uplink transmission bandwidth. Each such resource consists of 12 subcarriers, i.e., one resource block, within each of the two slots of an uplink subframe. In order to provide frequency diversity, these frequency resources may be shifted between different parts of the spectrum at the slot boundary, using so called frequency hopping, as illustrated in FIG. 5.
FIG. 5 shows an example of frequency hopping in the uplink, with one resource consisting of 12 subcarriers at the upper part of the spectrum within the first slot of a subframe and an equally sized resource at the lower part of the spectrum during the second slot of the subframe. If more resources are needed for the uplink L1/L2 control signaling, e.g. in case of very large overall transmission bandwidth supporting a large number of users, additional resource blocks can be assigned next to the previously assigned resource blocks.
The reasons for locating the PUCCH resources at the edges of the overall available spectrum are two-fold:                Together with the frequency hopping described above, the location at the edges maximizes the frequency diversity experienced by the control signaling, and        Assigning uplink resources for the PUCCH at other positions within the spectrum, i.e. not at the edges, would fragment the uplink spectrum, making it impossible to assign very wide transmission bandwidths to a single mobile terminal and still retain the single-carrier property of the uplink transmission        
The bandwidth of one resource block during one subframe is larger than the control signaling needs of a single terminal. Therefore, to efficiently exploit the resources set aside for control signaling, multiple terminals can share the same resource block. This is done by assigning the different terminals different orthogonal phase rotations of a cell-specific length-12 frequency-domain sequence and/or different orthogonal time-domain covers covering the subframes within a slot or subframe.
The LTE Release-8 standard supports bandwidths up to 20 MHz. In order to meet the IMT-Advanced requirements, bandwidths larger than 20 MHz need to be supported. However, one important requirement is to assure backward compatibility with LTE Release-8 for legacy terminals. This should also include spectrum compatibility. That would imply that an LTE-Advanced carrier should appear as a number of LTE carriers to an LTE Release-8/9 terminal. Each such carrier can be referred to as a component carrier (CC). FIG. 7 shows a schematic view illustrating five component carriers 702-710 of 20 MHz each, together forming an aggregated bandwidth 700 of 100 MHz. In particular for early LTE-Advanced deployments it can be expected that there will be a smaller number of LTE-Advanced-capable terminals as compared to the number of LTE legacy terminals in use. Therefore, it is necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. making it possible to implement carriers where legacy terminals can be scheduled in all parts of a wideband LTE-Advanced carrier. The straightforward way to obtain this would be by means of carrier aggregation. Carrier aggregation implies that an LTE-Advanced terminal can receive multiple CCs, where each CC has, or is at least able to have, the same structure as a Release-8 carrier.
Scheduling of a CC is done on the Physical Downlink Control Channel (PDCCH) via downlink assignments. Control information on the PDCCH is formatted as a Downlink Control Information (DCI) message comprising predetermined bit fields for different types of control information. DCI messages for downlink assignments contain, among other things, resource block assignment, modulation and coding scheme related parameters, hybrid-ARQ redundancy version, etc. In addition to parameters relating to the actual downlink transmission, most DCI formats for downlink assignments also contain a bit field for carrying Transmit Power Control (TPC) commands. These TPC commands are used to control the uplink power of the corresponding Physical Uplink Control Channel (PUCCH) that is used by terminals to transmit the hybrid-ARQ feedback.
The fact that an LTE-Advanced terminal could be assigned more resources than a legacy terminal, and on several component carriers, increases the need for control information, e.g. since more resources need to be addressed, and more feedback needs to be transmitted, as compared to a Release-8 scenario. For example, in Release-8 FDD, the number of ACK/NAK bits to be transmitted in the uplink as a response to a downlink assignment/transmission is limited to 1 bit for single code word, and 2 bits for dual code word transmission, while in Release-10, when a mobile terminal, also denoted UE (User Equipment), is assigned e.g. 3 downlink component carriers, the ACK/NAKs associated with these component carriers will require no less than 3 bits assuming single code word, and 6 bits assuming dual code word transmission on all component carriers, respectively. When the case when a mobile terminal does not receive any assignment(s) on one or multiple component carriers also shall be included in the feedback structure, the number of required feedback bits increases even further, to 5 and 7 bits, respectively, assuming again single code word and dual code word transmission, respectively, on all three component carriers. The event that an assignment, even though scheduled, is not received by a mobile terminal is referred to as DTX.
The component carriers can be of different bandwidths, e.g. 5, 10 or 20 MHz, and thus comprise different amounts of resources, which need to be addressed. A wide component carrier will therefore require more control bits for addressing the resources within the carrier than a comparatively narrow component carrier. These differences depending on the bandwidth of CCs or amount of assigned resources on each CC could be solved by using differently sized control messages, comprising an addressing space which corresponds to the current resource conditions. However, such a solution would require much processing of a receiver, in terms of blind detection of control messages.
In LTE, a mobile terminal has to blindly decode DCI control messages to establish if it is currently scheduled. To reduce the complexity, a mobile terminal may be instructed to only monitor, i.e. blindly decode, DCI message formats of certain payload sizes. Forcing a mobile terminal to monitor DCI formats with a large variety of payload sizes increases the number of blind decodings a mobile terminal has to perform and thus also the mobile terminal complexity.
In order to maintain or reduce blind detection, it is desirable to have control messages and e.g. address fields of equal size. This implies that an address field suitable for addressing the resources within a 5 MHz component carrier will be too scarce for addressing the resources within a 20 MHz component carrier, and an address field large enough to address the resources within a 20 MHz component carrier will be unnecessarily large for addressing the resources within a 5 MHz component carrier. Thus, a too small addressing space will only allow for a rough addressing in wide CCs, while a larger addressing space will waste resources when used for relatively narrow component carriers.
Consequently, it is a problem how to provide additional control information without increasing resource waste, causing insufficient addressing or increasing the burden of blind detection in a receiver.